1. Field of the Invention
The present invention generally relates to a frequency hopping apparatus and method in a wireless communication system, and in particular, to a frequency hopping apparatus and method in a Frequency Division Multiple Access (FDMA) wireless communication system.
2. Description of the Related Art
Wireless communication systems have been developed to provide communication regardless of positions of users. Each of the wireless communication systems distinguishes users with limited resources and performs communication with the users. There are various possible schemes which can be classified according to the method of using limited resources. For example, a scheme of distinguishing users with orthogonal code resources is referred to as Code Division Multiple Access (CDMA), a scheme of distinguishing users with time resources is referred to as Time Division Multiple Access (TDMA), and a scheme of distinguishing users with frequency resources is referred to as Frequency Division Multiple Access (FDMA).
Each of the foregoing schemes can be subdivided into various types, and more than two schemes can be used together. For example, as for FDMA, a method of allocating orthogonal frequency resources to every user in a particular manner and performing communication with the user is referred to as Orthogonal Frequency Division Multiple Access (OFDMA). Therefore, the OFDMA method is one of the FDMA methods. Each of the various systems allocates its system resources to users and performs communication with the users. A description will now be made of resource allocation with reference to, for example, an OFDMA system.
FIG. 1A shows an application of hopping in an OFDMA system according to the prior art. OFDMA is a system that supports multiple access by allocating different orthogonal sub-carriers to users.
Referring to FIG. 1A, the horizontal axis indicates the time axis, and the vertical axis indicates the frequency axis. A description will now be made of each part shown in FIG. 1A. One rectangle denoted by reference numeral 101 indicates one block. That is, in OFDMA, resources are allocated in units of the blocks, user data is encoded into a coded sequence according to a predetermined rule, and the one coded sequence is transmitted through one or multiple blocks.
FIG. 1B shows a structure of one block according to the prior art. With reference to FIG. 1B, a description will now be made of one block.
Referring to FIG. 1B, one block is shown which is a basic resource allocation unit in a general OFDMA system. Reference numeral 101-1 denotes one sub-carrier in the frequency axis, and reference numeral 101-2 denotes one slot in the time axis. That is, the slot is a basic transmission unit in the time axis, and one slot generally includes multiple OFDM symbols. In FIG. 1B, a length of a small square in the time axis is a length of one OFDM symbol. As described above, one block is composed of multiple sub-carriers and multiple OFDM symbols. In the foregoing example, one block is a 2-dimensional square which is composed of 8 sub-carriers in the frequency axis and 10 OFDM symbols in the time axis. Therefore, 8×10 modulation symbols can be transmitted in one block. The parts denoted by reference numeral 101-3 indicate the positions where pilot symbols are transmitted, and the other parts denoted by reference numeral 101-4 indicate the positions where data symbols are transmitted. That is, coherent detection is achieved depending on channel estimate values obtained using the pilot symbols in a demodulation process for the data symbols transmitted on the one block. Data is demodulated through the coherent detection. The block formed as shown in FIG. 1B is one square in FIG. 1A.
Although a localized block, elements of which are localized in one block, has been described in the foregoing example, a distributed block, elements of which are distributed over several blocks, is also possible. In FIG. 1A, numerals denoted by reference numeral 102 are identifiers for Transmission Time Intervals (TTIs). One TTI, denoted by reference numeral 104, is a time unit for which a coded sequence for one data packet is transmitted, and one TTI is generally composed of one or multiple slots. In the foregoing example, one slot is composed of one TTI. Numerals denoted by reference numeral 103 are identifiers for blocks in the frequency axis, and as shown by reference numeral 105, shaded squares indicate one logical channel. The logical channel is a resource allocation unit, and a phrase ‘allocation of a logical channel #1 to a particular user’ indicates a process of allocating specific blocks predefined between a transmitter and a receiver to the user. In the channel allocation, the term ‘logical’ is used for the following reasons. The system, to which frequency hopping (hopping) is applied, defines the logical channels rather than physically defining channels and allocating the physical channels. This is because it is convenient to define mapping between the logical channels and physical channels using a specific hopping sequence.
Hopping, as used herein, means an operation in which squares corresponding to the logical channel #1 of FIG. 1A continue to change with the passage of time. Hopping is effective when it is applied, particularly in cellular systems. This is because the same logical channel #1 is allocated to different users between two adjacent cells and different hopping patterns are defined between cells, thereby contributing to randomization of inter-cell interference.
Generally, a mobile communication system uses Hybrid Automatic Repeat reQuest (HARQ) in transmitting packet data. HARQ is an important technology used for increasing data transmission reliability and data throughput in a packet-based mobile communication system. HARQ refers to a combined technology of Automatic Repeat reQuest (ARQ) technology and Forward Error Correction (FEC) technology. ARQ is a technology which is popularly used in wire/wireless data communication systems. In the ARQ technology, a transmitter transmits a transmission data packet with a sequence number assigned thereto according to a predefined rule, and a data receiver sends to the transmitter a retransmission request for a missing packet among the received packets using the sequence number, thereby achieving reliable data transmission. FEC means a technology for adding redundant bits to transmission data according to a specific rule, such as convolutional coding, turbo coding, etc., to cope with noises generated in a data transmission/reception process and/or errors occurring in fading environments, thereby demodulating the transmitted original data. In a system using HARQ realized by combining the two technologies, i.e. ARQ and FEC, a data receiver performs a Cyclic Redundancy Check (CRC) check on the data decoded through an inverse FEC process on the received data, to determine presence/absence of an error in the received data. In absence of error, the data receiver feeds back an Acknowledgement (ACK) to a transmitter so the transmitter may transmit the next data packet. However, if it is determined from the CRC check result that there is an error in the received data, the data receiver feeds back a Non-Acknowledgement (NACK) to the transmitter, requesting retransmission of the previously transmitted packet. In this process, the receiver combines the retransmitted packet with the previously transmitted packet to obtain energy gain, thereby obtaining highly improved performance compared with the conventional ARQ not having the combining process.
FIG. 2 shows resource allocation in a system to which HARQ is applied according to the prior art. The parts denoted by reference numerals 201, 202, 203 and 204 are similar to corresponding parts in FIG. 1A. Interlace indexes denoted by reference numeral 205 are identifiers for TTIs in which one HARQ operation is performed in the time axis. That is, in the system to which HARQ is applied, packet data transmission is performed in the TTIs corresponding to the same interlace identifier. For example, FIG. 2 means that TTI identifiers corresponding to an interlace #0 are 0, 6, 12, 18, . . . , and independent packet data is transmitted/received for individual interlaces. In this example, there are 6 interlaces #0, #1, #2, #3, #4 and #5. Generally, the number of interlaces is determined during system design taking into account a data demodulation time and a transmission time of ACK/NACK 206. In the foregoing description, a time interval between interlaces, i.e. a time interval between retransmissions for one packet for one HARQ operation, is defined as an HARQ Round Trip Time (RTT) 208.
The hopping described in FIG. 1A is disadvantageous in that it may decrease in channel estimation performance because a position of the same logical channel changes. More specifically, as shown in FIG. 1B, channel estimation should be performed through specific pilot symbols in the block. However, in a system not supporting the hopping, i.e. when one logical channel includes the same block index in the time axis, channel estimation performance can be improved by using more pilot symbols included in several consecutive blocks in the channel estimation process. On the contrary, a system to which the hopping is applied should perform channel estimation only with pilots included in one block, causing a decrease in its performance. However, if HARQ is applied without the hopping as shown in FIG. 2, HARQ performance may decrease. For example, in FIG. 2, a packet initially transmitted at TTI #0 may fail in successful demodulation at a receiver, when a position of the block corresponds to the position having a poor channel environment in the frequency axis or having considerable interference from other cells. Therefore, it is preferable to attempt transmission over different blocks during retransmission to obtain diversity gain. In other words, performing retransmission in the same position in TTI #6, as shown in FIG. 2, may have difficulty in obtaining the diversity gain.